1. Field of the Invention
The present invention involves techniques relating to a display device comprising an EL (electro luminescence) element (hereinafter referred to as an EL display device) and to a display that uses the EL display device.
2. Description of the Related Art
Studies of EL display devices comprising EL elements as self-light emitting elements have flourished in recent years. In particular, organic EL display devices using organic materials for EL materials have held the attention. The organic EL display devices are also called organic EL displays (OELDs) or organic Light Emitting Diodes (OLEDs).
EL display devices are of self-light emitting type, unlike liquid crystal display devices, and therefore the view angle does not matter in the EL display devices, which forms one of their characteristics. That is, the EL display devices are more suitable for displays used outdoors than the liquid crystal display devices, presenting so many possible ways of their use.
EL elements have the structure in which one EL layer is sandwiched between a pair of electrodes. An EL layer usually has a layer structure. As a typical example thereof, a layer structure consisting of hole transmitting layer/light emitting layer/electron transmitting layer which has proposed by Tang, et al. from Kodak Eastman company can be named. This is so highly efficient in terms of light emission that most of EL display devices whose research and development is now under way employ that structure.
Light is emitted by applying a given voltage generated between the pair of electrodes to the EL layer having the above structure to cause re-combination of carriers in the light emitting layer. A method to achieve this is chosen from two options in which one is to form the EL layer between two kinds of stripe-like electrodes arranged perpendicular to each other (simple matrix method) and the other is to form the EL layer between pixel electrodes that are connected to TFTs and are arranged in matrix and opposite electrodes (active matrix method).
Both methods requires to form the EL layer on the electrodes that have been patterned out. The EL layer, however, is easily influenced and degraded by changes such as level differences, so that various kinds of contrivance are made to improve the flatness. This leads into a problem of complication of manufacturing process and, accordingly, an increase in production cost.
The present invention has been made in view of the problem above, and an object of the present invention is therefore to provide a method of manufacturing an EL display device of high definition by uncostly measures. Another object of the present invention is to provide an inexpensive electronic device having the EL display device as such.
According to the present invention, a light emitting region is distinguished from a non-light emitting region by a totally novel method which has not been found in prior art. Specifically, the novel method is a technique characterized in that a light emitting layer is selectively doped with a specific impurity element to cause selective light emission from the region doped with the impurity element. In this specification, the term specific impurity element means an impurity element that is capable of making the light emitting layer function as a hole transmitting layer (or a hole injection layer) or an electron transmitting layer (or an electron injection layer) when used to dope the light emitting layer.
The present invention includes three methods as follows: a first method is to dope into the vicinity of the interface between an anode and a light emitting layer with a specific impurity element, a second method is to dope into the vicinity of the interface between a cathode and a light emitting layer with a specific impurity element, and a third method is to dope into both the vicinity of the interface between the anode and the light emitting layer and the vicinity of the interface between the cathode and the light emitting layer with different specific impurity elements.
The first method is characterized in that the vicinity of the interface between the anode and the light emitting layer is doped with a halogen element as a specific impurity element, typically, F (fluorine), Cl (chlorine), B (bromine) or I (iodine). The vicinity of the interface between the anode and the light emitting layer refers to an extent 100 nm (50 nm, typically) down the depth of the light emitting layer from the interface between the anode and the light emitting layer. No trouble is caused if the halogen element is contained in the anode.
The second method is characterized in that the vicinity of the interface between the cathode and the light emitting layer is doped with, as a specific impurity element, an alkali metal element, typically, Li (lithium), Na (sodium), K (potassium) or Cs (cesium), or an alanine earth metal element, typically, Be (beryllium), Mg (magnesium), Ca (calcium) or Ba (barium). The vicinity of the interface between the cathode and the light emitting layer refers to an extent 100 nm (50 nm, typically) down the depth of the light emitting layer from the interface between the cathode and the light emitting layer. No trouble is caused if the alkali metal element or an alkaline earth metal element is contained in the cathode.
The third method is characterized in that it is a combination of the first method and the second method, and in that the vicinity of the interface between the anode and the light emitting layer is doped with a halogen element as a specific impurity element, while the vicinity of the interface between the cathode and the light emitting layer is doped with, as a specific impurity element, an alkali metal element or an alkalin earth metal element.
A known doping method suites the doping of the above specific impurity element. Ion doping that does not involve mass separation, ion implantation that involves mass separation, vapor phase doping that utilizes diffusion are preferable. Whichever method is used, it is whether the method allows selective doping of the above specific impurity element that matters.
According to the present invention, the vicinity of the interface between the anode, or the cathode, and the light emitting layer is selectively doped with a specific impurity element, and only the portion doped with the impurity element emits light when the voltage is applied. In other words, drive voltage of an EL element in the present invention is adjusted such that the light emitting layer emits no light by itself, or emits light of extremely low luminance. Further adjustment is made so that the portion doped with the specific impurity element emits light of satisfiable luminance at the same drive voltage.
That is, the present invention is characterized by doping the light emitting layer with a specific impurity element to use the doped portion as a hole transmitting layer or an electron transmitting layer in any of the first, second, and third methods. This makes a drive voltage, which is too low to cause substantial light emission of the light emitting layer by itself, sufficient for only the portion doped with the specific impurity element to emit light of satisfiable luminance.